Method of using a laminated composite radius filler

ABSTRACT

A laminated composite (multi-ply) radius filler includes a plurality of woven fabric reinforced plies cut to fill a radius gap (including, if appropriate, intentional overfill) to increase absolute strength, to increase specific strength, or to reduce cost by reducing cracking and distortion in the radius of a composite assembly. The present invention also describes the method of manufacture and method of use for such laminated composite radius filler.

REFERENCE TO RELATED APPLICATION

[0001] The present application is a divisional application based uponU.S. patent application Ser. No. 09/793,810, filed Feb. 23, 2001, whichclaims the benefit of U.S. Provisional Patent Application No.60/184,871, filed Feb. 25, 2000.

TECHNICAL FIELD

[0002] The present invention relates to a laminated composite radiusfiller, its method of manufacture, and its method of use, especially inmaking high performance, high quality lower cost, aerospace compositeassemblies.

BACKGROUND ART

[0003] Failure of composites, especially in aerospace applications,often initiates in the resin in the radius filler (i.e., “noodle”) thatfills the interface between plies in laminated joints. Cracks in theradius filler can be formed into the composite during manufacture (e.g.,improper tooling, improper handling of tools, or residual strain), cangrow from voids that provide a nucleation site for crack growth, or canarise when structural loading overstresses the resin. Residual tensilestrain is often designed into composites today, and arises from mismatchin the coefficient of thermal expansion between the radius filler andthe surrounding structure, especially the reinforcing fibers, or fromshrinkage of the resin that arises during cure of the composite.

[0004] Composite spars or ribs are made by binding two “C” or “U”channels together to form a web with flanges. The channels generallycomprise a plurality of plies of fiber-reinforced resin, commonly in theform of prepreg. The fiber reinforcement might be unidirectional tape orwoven fabric, and, most commonly, is carbon fiber or fiberglass. Foraerospace structure, it normally is woven carbon fiber fabric. Thefabric usually is not isotropic with respect to its reinforcementstrength. It may be easier to stretch or to expand the fabric in itswidth rather than in its length. In the different plies, the fabric canbe oriented in different directions, specified as an angle of rotationfrom a reference direction. That is, the orientation might be 0° or +45°or −45° or 90°, although other orientations are sometimes used. Here,“+45°” might mean that the fabric is rotated 45° clockwise while “−45°”might mean a 45° rotation in the counterclockwise sense. Ply orientationin the laminate evens the strength or impact resistance making thecomposite more uniform or less angle dependent. Oriented composites maybe extremely strong in the direction of unidirectional reinforcingfibers while being relatively weak perpendicular to those fibers.

[0005] The plies are bent in a predetermined radius to form the “C” or“U” channel. When the channels are joined at the webs, a dimple occursalong the flange because of these radii. A radius filler fills thedimple. (See FIG. 1 or 11.) Using a radius filler prevents distortionthat otherwise would occur when the spar or rib were loaded with abending or twisting moment. Distortion can reduce the strength of thecomposite significantly and can also increase part variability (i.e.,the spars simply are not the same shape from part to part).

[0006] Existing designs for radius fillers have produced fillers thatare structurally inadequate, that are challenging and expensive toproduce, or that leave the structural integrity of the resultingcomposite in question. Such designs often force post-manufacturing,non-destructive evaluation (NDE) and inspection (NDI), which slowsproduction flow, increases cycle time, and increases cost. Therefore,there is a need for an improved radius filler that is easy andinexpensive to manufacture and structural sound to prevent distortion.The radius filler of the present invention allows the production ofstronger, higher quality composites with lower variability whileimproving flow and cycle time and simultaneously reducing the overallcomposite cost.

[0007] When cracks cannot be avoided through a robust design as nowachievable with the radius filler of the present invention, thestructure needs to be made larger and heavier than optimal to withstandthe design loads. Performance or payload in the aircraft is diminishedbecause of the larger, heavier parts. Higher costs are also incurredboth in its production and use.

[0008] Designers would like to build parts where performance of theradius filler is challenged even more severely than in existing, fieldedaircraft. That is, designers would like the radius fillers havingincreased structural properties to withstand even greater stresses andpull-off loads than is achievable today. Such a radius filler wouldallow higher performance wings to be built. Therefore, absolute strengthof the composite assembly is important. The radius filler of the presentinvention provides higher absolute strengths than are achievable withexisting radius fillers. Therefore, the radius filler of the presentinventions expands the domain of acceptable composite designs that canbe used to meet aerospace challenges.

SUMMARY OF THE INVENTION

[0009] A laminated composite radius filler “a noodle” of the presentinvention better meets the challenges faced with composite design byreducing the initiation of processing (manufacturing) induced cracks orpremature cracking of composite assemblies, like a spar orskin-stiffener interface, under structural loading. That is, the“noodle” no longer is the weakest link in the composite structure or, ifit remains the weakest, it still has a higher absolute strength thanprevious radius fillers allowed.

[0010] The present invention relates to a laminated composite radiusfiller having higher resistance to distortion, to its method ofmanufacture, and to its method of use. The radius filler permits designand manufacture of composite structures, like spars, ribs, orskin/stiffener assemblies, that have higher resistance to distortion,higher absolute strength, increased specific strength (i.e., strengthper unit weight), lower part variability, and lower production cost. Theradius filler enables the manufacture of stronger while lightercomposite structure, which enables improved wing or other airfoildesign. Cracking failure in the radius filler is reduced and pull-offstrength is increased.

[0011] The present invention relates to the radius filler, to its methodof manufacture, to its method of use, and to products that use it. Apreferred radius filler of the present invention has a laminated fiberbody and a unidirectional tip. The laminated fiber body typically hastwo distinct sections that are trapezoidal in cross-section. The uppersection, for example, may have 14 or 18 plies of IM7/5250-4 thin tapewith ±45° orientation (i.e., the plies alternate from having a ±45°orientation relative to the X-axis and −45° orientation). The lowersection is made from the same material but has 10 plies. Typically thenoodle is completed with three additional, base plies. The number ofplies and sections are selected to configure the radius filler to theshape of the dimple. To simplify the discussion, this description willfocus on substantially triangular radius filler for T-section joints(such as joint between a stiffener and a skin), but other configurationsfor the dimple and the radius filler are possible.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a sectional view of a typical wingbox in aerospaceconstruction showing a rib or spar joined to upper and lower skins. AT-section at the skin/stiffener interface of the rib or spar with theskin includes a radius filler.

[0013]FIG. 2 is a photomicrograph of a radius filler havingunidirectional fiber reinforcement in the X-direction identified inFIG. 1. This figure also shows the plies of the spar and skin.

[0014]FIG. 3 is another photomicrograph similar to FIG. 2 showing acrack in a unidirectional noodle of the type shown in FIG. 2.

[0015]FIG. 4 is yet another photomicrograph, similar to FIG. 2, showinga preferred embodiment of a radius filler of the present invention in aspar-skin interface.

[0016]FIG. 5 shows a cross-section of ply stacks of the radius filler ofFIG. 4.

[0017]FIG. 6 shows another preferred laminated composite (multi-ply)radius filler of the present invention having a triangularcross-section.

[0018]FIG. 7 shows another preferred laminated composite (multi-ply)radius filler of the present invention having sidewalls of a generallytriangular shape curved to match roughly the intended radius.

[0019]FIG. 8 is an isometric of the radius filler of FIG. 6 formed intoa sine wave shape.

[0020]FIG. 9 illustrates cutting a radius filler of FIG. 6 from adebulked laminate using an ultrasonic knife carried on a sonotrobe hornon an American GFM ultrasonic cutting table.

[0021]FIG. 10 shows a typical layout in plan view for cutting the radiusfillers of FIG. 6 from a laminate.

[0022]FIG. 11 shows the radius filler of FIG. 6 installed in the gapcaused by the curvature of joined “C” channels in a typical spar.

[0023]FIG. 12 shows the preferred layup and processing thermal cycle toprepare the laminate of FIG. 10 in preparation for cutting radiusfillers of the present invention.

[0024]FIG. 13 shows a typical ply arrangement for forming a radiusfiller of the type shown in FIGS. 4 and 5.

[0025]FIGS. 14, 15, and 16 show the sequence of debulking a radiusfiller of the type shown in FIG. 13, to prepare it for use.

[0026]FIGS. 17 and 18 show the details of a 3-ply base for a radiusfiller and its installation, in the debulking operation.

[0027]FIG. 19 shows the bagging sequence for the debulking sequenceshown in FIGS. 14-18.

DETAILED DESCRIPTION

[0028] As shown in FIG. 1, a spar 10 has a left 12 and right 14 “C”channel bonded together. The spar is bonded through flanges 16 definedby the “C” channels 12 and 14 and, optional, facing plies 18 to an upper20 and lower 22 skin in an aerospace composite assembly typical of awing, vertical stabilizer, horizontal stabilizer, or the like. The “C”channels in the spar 10 have opposite curvatures that create a dimplewhen the channels are bonded together. A radius filler 24 fills thevolume of the dimple. The web of the spar 10 can be straight, but oftenin fighter aircraft it is shaped like a sine wave to increase itsstrength. The radius filler 24 assumes the geometry of the dimple. For asine wave spar, the radius filler 24 is also a sine wave (FIG. 8).

[0029] The spar 10 and skins 20 and 22 are fiber reinforced resin matrixcomposites. In our tests, we used BMS 5250-4 BMI (bis-maleimide)thermosetting resin reinforced with carbon fiber fabric with the spar 10co-cured to the skins 20 and 22 radius fillers 24 of the presentinvention were made with BMS 5250-4 BMI thin tape with a designedoverfill of 115%. We have observed little drop in strength for smallerradius fillers so long as the radius filler at least substantially fillsthe dimple. We also have observed some strengthening if greateroverfill, up to as large as 190%, is used. The strength is relativelyinelastic to variations in the dimensions of the radius filler withinthese broad boundaries.

[0030]FIGS. 2 and 3 show a common radius filler made by pultrudingrolled unidirectional tape, like BMS 5250-4 thin tape in a die or byextruding or pulling the rolls through shaped roller dies. While theshape we illustrate in this application has a triangular cross-section,the radius filler can assume other shapes, like a diamond or star, shownin U.S. Pat. No. 5,833,786, which we incorporate by reference.Unidirectional radius fillers suffer from cracking 26, as shown in FIG.3. Pure resin adhesive radius filler also are plagued with cracking.

[0031]FIG. 4 shows one embodiment of the laminated composite radiusfiller 24 of the present invention having a laminated fiber body 28 anda unidirectional tip 30. The laminated fiber body has two distinctsections that are trapezoidal in cross-section. The upper section 32 has14 or 18 plies of IM7/5250-4 thin tape with ±45° orientation (i.e., theplies alternate from having a ±45° orientation relative to the X-axis(FIG. 1) and −45°). The lower section 34 is made from the same materialbut has 10 plies. Typically the noodle is completed with threeadditional plies 36 (FIG. 13). The number of plies and sections areselected to configure the radius filler to the shape of the dimple.While we recommend a ±45° orientation, other ply arrangements can beused to introduce 0° or 90° plies or even plies at some other angle. The±45° orientation provides a radius filler having adequate strength withrelative ease of bending into a sine wave configuration (FIG. 8) orother shape. 0° or 90° plies impact the ability to bend the radiusfiller.

[0032] We determined that elegant matching of the radius filler shape tothe shape of the dimple was unnecessary. We also observed that overfillwithin a reasonable range improved performance. Therefore, our preferredradius filler today has a simple triangular cross-section 38 withthirty-eight plies at ±45° orientation for filling a 0.20-inch radius.The radius filler of FIG. 6 is a replacement for the tip-and-body radiusfiller of FIG. 4.

[0033] If the overfill from using a triangular cross-section will beexcessive (and we question whether it can be if you can squeeze thenoodle into the dimple), you might shape the radius filler 40 as shownin FIG. 7 using planes, gouges, or other cutting tools. We preferstraight sides in an isosceles right triangle because this shape can beeasily cut from debulked laminate using an ultrasonic cutting table (SeeFIG. 10). FIG. 1 shows proper installation of the radius filler 38 inthe dimple. FIG. 12 illustrates the apparatus and heat-pressure cyclefor debulking the radius filler. FIGS. 14-19 illustrate the debulkingsequence in greater detail. As shown in FIG. 12, the operation isrelatively simple: vacuum bag the plies on a mandrel using an FEPrelease film to protect against sticking the radius filler to themandrel. Then, in an autoclave or other suitable pressure vessel, exposethe “green” radius filler to 85 psi pressure at 200° F. for 60 minuteswith 5° F./min heat up and cool down temperature ramps. This cycle isused to debulk the sections 32 or 34 or the entire radius filler 38prior to ultrasonic cutting.

[0034] We lay up the plies in accordance with BPS 5PTPLB01-C, hot debulkper PTS 98 PTS-001, Rev. A, and ultrasonically trim using a GFM US-50cutter. We place one ply of porous FEP (fluorinated ethylene propylene)film between the “green” radius filler and the lay up mandrel (table).The cutter uses a GFM UK-72 Cl.0 knife carried on a sonotrobe horn at5000 mm/min (about 550 in/min) with 30-40% amplitude and a 30° leadangle.

[0035] To make the multistacked laminated radius filler of FIG. 4, placethe tip 30 on FEP release plies in the noodle tool 43 (FIGS. 14 and 15).Then, position the body sections 32, 34, and 36 (FIGS. 16, 17 and 18)before bagging the lay up under a solid FEP film 45, caul plate 47,breather 49, and vacuum bag 50 (FIG. 19) for the hot debulk previouslydescribed.

[0036] The preferred radius filler (unitary or multistack) is easy tomanufacture, affordable, and robust. This radius filler actually iseasier to make (less time consuming than the common unidirectionpultrusion radius filler).

[0037] When cutting the laminates with the ultrasonic cutter, we mightmask the laminate with masking tape to provide adequate stiffness.

[0038] Table 1 compares the pull off strength between a co-cured skinand spar for a unidirectional radius filler like those shown in FIGS. 2and 3, with a laminated radius filler of the present invention. Thestrengths were measured using two-foot spar specimens. TABLE 1Unidirectional Laminated Radius Filler Radius Filler Average Pull OffStrength 1275 lb/in 1610 lb/in Standard Deviation 276 lb/in 125 lb/inCoefficient of Variation 21.6% 7.7% (COV) Population 23 24

[0039] Table 2 shows similar test results this time using seven-footspar specimens. TABLE 2 Unidirectional Filler Laminated Filler AveragePull Off Strength 1285 lb/in 1544 lb/in Standard Deviation 386 lb/in 102lb/in Coefficient of Variation 30% 6.6% Population 5 8

[0040] These results demonstrate a 20% to 25% improvement in pull offstrength for laminated radius filler in co-cured joints compared to thesame structure with a unidirectional radius filler. A two to three-foldreduction in variability has also been demonstrated. The seven-foot sparis representative of the size and type used in military aircraftstructure. The laminated radius filler was robust relative to scale upbased on the comparison of variability between the two and seven-footspars.

[0041] Table 3 compares the pull off strengths for laminated radiusfillers made by hand or with an automated technique similar to whatwould be used to make the radius filler for actual production ofcomposite assemblies. Automating the manufacture did not significantlyimpact performance of the radius filler. TABLE 3 Hand Laid AutomatedLaminated Filler Laminated Filler Average Pull Off Strength 1585 lb/in1598 lb/in Standard Deviation 131 lb/in 118 lb/in Coefficient ofVariation 8.3% 7.4% Population 12 19

[0042] Table 4 shows performance of several variants of the radiusfiller of the present invention. These test results confirm that theradius filler concept provides radius fillers having adequate strength.Also, the desired strengths can be achieved repeatedly. The radiusfiller performs consistently so that the composite assembly has astrength that is predictable by design and confirmed in testing within anarrow range of variation. TABLE 4 14 Multi- Drape X-Noodle 10 10A 14Manual Stack U Drape L R1 1581 1831 1403 1731 1739 1481 R2 1585 15471419 1656 1463 R3 1554 1736 1532 1705 1616 1600 R4 1472 1753 1580 15961859 1562 Average 1548 1717 1484 1672 1617 Std. Dev. 52.5 120.5 86.259.4 140.8 COV % 3.4 7.0 5.8 3.6 8.7

[0043] Typically, the radius filler is made from the same resin andfiber used in the spar and skin.

[0044] In some cases the tip 30 can be eliminated because the volume itfills is so small.

[0045] The ply orientation can be selected so that the coefficient ofthermal expansion (CTE) for the radius filler closely matches that ofthe bulk composite. CTE matching reduces the possibility of the radiusfiller cracking from thermal stresses. The ply orientation can also beadjusted to obtain the optimum modulus for joint performance. The pliesextend fibers across the region where cracks normally form when usingconventional unidirectional noodles.

[0046] Because the preferred radius filler fills or overfills thedimple, there is less likelihood of resin depletion occurring bybleeding of resin from the surrounding structural plies.

[0047] If desired, some plies in the body sections can useunidirectional tape.

[0048] Our test data shows an increase in strength of about 20-25%;consistent, repeatable performance; and a 2 or 3-fold reduction invariability.

[0049] We estimate that the laminated composite radius filler willreduce the cost of a national fighter wingbox structure by 15% and willreduce the weight of the wingbox by 100 pounds in a co-cured unitizedstructure. A lighter structure permits higher performance, longer range,or a combination of both. A reduced fastener count eases assembly,improves survivability, and reduces maintenance costs.

[0050] While we have described preferred embodiments, those skilled inthe art will readily recognize alternatives, variations, andmodifications that might be made without departing from the inventiveconcept. Therefore, interpret the claims liberally with the support ofthe full range of equivalents known to those of ordinary skill basedupon this description. The examples are given to illustrate theinvention and not intended to limit it. Accordingly, limit the claimsonly as necessary in view of the pertinent prior art.

We claim:
 1. A method for using a radius filler to improve absolutestrength to improve specific strength, to reduce cracking, to reducecost, or to reduce distortion and variability in a composite assembly,comprising the steps of: (a) positioning a radius filler of the typehaving a laminate of debulked plies of fabric reinforced compositesubstantially the shape of the radius gap that the radius filler fillsand having at least one ply oriented at +45° and another ply oriented at−45° relative to the longitudinal axis of the filler in a radius gap ina composite assembly, the radius filler having a resin that is uncuredor partially cured; and (b) completing the curing of the radius fillerto complete the composite assembly.
 2. A method for improving crackresistance and reducing variability in composite assemblies having abulk composite, comprising the step of: (a) using a laminated compositeradius filler having optionally, a composite tip in the form of a wedgemade with unidirectional fibers; (b) a laminate of plies of fabricreinforced composite substantially the shape of the radius gap that theradius filler fills and having at least one ply oriented at +45° andanother ply oriented at −45° relative to the longitudinal axis of thefiller in a radius gap.
 3. The method of claim 2 wherein the radiusfiller is oversized.
 4. The method of claim 2 further comprising thestep of selecting the ply orientation so that the coefficient of thermalexpansion for the radius filler closely matches that of the bulkcomposite to reduce cracking from thermal stresses.
 5. A compositeassembly that is the product-of-the-process of claim
 2. 6. A compositeassembly that is the product-of-the-process of claim 4.